1. Field of the Invention
The present invention relates to a supported Cobalt-based catalyst, which can be used in the Fischer-Tropsch reaction in a gas-liquid-solid fluidized reactor.
2. Description of the Background
The activity of Fischer-Tropsch catalysts can be influenced by all physical factors which affect the mass transfer rate of the reagents and products, between the various phases and heat transfer. As a result, it can be observed that under diffusive regime conditions, there is not only a lower reaction rate but also a change in the selectivity of the various products, jeopardizing the performance of the whole process both from a quantitative and qualitative point of view.
In a catalytic reaction, the transfer processes of material and heat between fluid and catalyst depend on the fluid-dynamic regime to which the reagents and reaction products are subjected and on the geometry of the reactor, i.e. the type of reactor solution adopted.
In the Fischer-Tropsch synthesis fixed bed reactors, entrainment reactors, fluidized bed reactors can be used, as described in the patent U.S. Pat. No. 4,670,472 (Dyer et al.). More recently gas-liquid-solid (slurry bubble column) fluidized systems are preferred to other reactor solutions. The flow-rates of the fluids in these solutions must be such as to guarantee a more or less homogeneous suspension of the catalyst in the entire reaction volume and to facilitate the removal of the heat produced by the exothermic reaction, improving the heat exchange between reaction zone and a suitable exchanger introduced into the column. As far as the catalyst is concerned, the solid particles must have sufficiently large dimensions as to be easily separated from the liquid products, but sufficiently small as to minimize intra-particle diffusion limitations.
Limitations to transport processes (of matter and/or heat) can be subdivided into external (or inter-particle) diffusion regime and internal (or intra-particle) diffusion regime. The entity of the phenomenon linked to external diffusion depends on the fluido-dynamics and geometry of the system, i.e. on the rate and characteristics of the reagent fluid and interphase surface area (form and dimensions of the catalyst particles). Internal diffusion phenomena, on the other hand, depend on the chemical and morphological structure of the catalyst (pore dimensions, surface area, density of the active sites) and on the molecular dimensions of the species in question.
Multiphase reactors of the slurry type generally use small catalyst particles (20-150 xcexcm) which do not give internal diffusion problems but can be subject to external mass transfer limitations owing to the low diffusion of the gases in the hydrocarbon liquid and relatively low fluid rate. On the contrary, the relatively high thermal conductivity of the liquid allows external heat transfer limitations to be ignored (J. M. Smith, xe2x80x9cChemical Engineering Kineticsxe2x80x9d, McGraw Hill Int. D., 1988, chap. 10, page 415).
Internal transport phenomena, on the other hand, are linked to the morphological parameters of the porous material used as carrier of the active phase, which determine the diffusion capacity inside the catalyst particle. The effect of intra-particle transport limitations is to generate a negative concentration gradient of the reagents inside the catalyst particle which, as final effect, causes a drop in the reaction rate.
In the same way it is possible to observe temperature gradients which, due to an exothermic reaction, such as the Fischer-Tropsch synthesis, create a temperature rise towards the center of the catalyst particle increasing its reaction rate, thus, with an effect contrary to mass transfer, favoring the selectivity to light hydrocarbons. Also in the case of reactions with a decrease in the number of moles, total pressure gradients are produced, capable of creating reagent streams towards the particle center. While on the one hand, this phenomenon increases the diffusion of the reagents towards the inside of the catalyst, on the other hand it delays the diffusion of the reaction products towards the outside.
In a multiphase reaction such as the Fischer-Tropsch synthesis, the transport processes of the reagents and products are conditioned by the presence of the hydrocarbon liquid produced. More specifically, the different diffusivities of the reagents (CO, H2) in the hydrocarbon liquid, approximately a 103-104 factor lower with respect to the diffusivities in the gas, create low concentrations of CO towards the center of the particle with a consequent progressive rise in the H2/CO ratio inside the catalyst. This condition favors the formation of light hydrocarbons and secondary reactions of the main products. From studies presented in literature in this field, it can be observed how, for catalysts based on differently supported cobalt used in the Fischer-Tropsch synthesis, it is possible to neglect internal diffusional limitations by operating with particles having a diameter of less than 200 xcexcm) (Iglesia, et al., Computer-aided Design of Catalysts, ED. Becker-Pereira, 1993, chap. 7).
In more general terms, for any catalytic reaction, internal transport phenomena become less important with the decrease in the catalyst particle dimension. For example for fluidized-bed or slurry applications, intra-particle heat transport limitations are generally negligible (J. M. Smith, xe2x80x9cChemical Engineering Kineticsxe2x80x9d, McGraw-Hill Int. D., 1988, chap. 11, page 451).
The ideal case in which there can be a total absence of mass and heat transport limitations is represented by homogeneous catalysts. These homogeneous systems however are not applied in many processes, owing to the difficulties and costs relating to the separation of the catalyst from the reaction medium. These costs, in fact, are often higher than the benefits deriving from the absence of diffusion limitations.
The catalyst particle dimensions are therefore of fundamental importance and must be sufficiently small so as to avoid constrictions to the mass and heat transport due to internal diffusion limitations and at the same time sufficiently large as to be easily separable from the suspension liquid.
The use of a slurry bubble column reactor (SBCR) in a gas-liquid-solid multiphase system in the Fischer-Tropsch synthesis is among the preferred reactor solutions. More specifically, in an SBCR, the catalyst is suspended in a hydrocarbon liquid, often the reaction product itself. The synthesis gas, consisting of CO, H2, N2, CO2, is fed by means of a suitable distributor capable of generating gas bubbles dispersed inside the suspension. The gas bubbles migrate upwards towards the slurry, being subjected to coalescence and breakage phenomena. In this way a wide distribution of the bubble diameters is created (3-80 mm), which determines the mixing and distribution of the catalyst inside the bubble column. The effectiveness of the mixing, and therefore the dispersion degree of the solid in the liquid, is mainly linked to the entrainment of the large gas bubbles (20-80 mm) with a rate of about 1-2 m/s.
The gaseous products are sent towards the top of the reactor and then processed externally, whereas the liquid products are recovered by filtration of the catalyst.
In spite of the advantages acknowledged in the use of an SBCR in the Fischer-Tropsch reaction (see references present in U.S. Pat. No. 5,939,350, col. 2) the aspects relating to the filtration are a critical point for the use of the whole process, owing to the reduced average particle dimensions of the solid used. To facilitate the filtration operations, it is therefore necessary to operate with catalyst particles having a sufficiently large diameter. The highest limit of the average particle diameter obviously depends on the catalytic performances obtained, which, as already mentioned, must not be affected by limitations of the diffusional type, capable of reducing the effectiveness of the catalyst with respect to what would be obtained in kinetic regime.
The innovative element of the present invention relates to the identification of a carrier with suitable morphological characteristics as to make it appropriate for application in a gas-liquid-solid fluidized reactor (SBCR) capable of simplifying the liquid/solid separation phase as a result of the favorable particle dimension of the carrier itself, and not negatively influencing the effectiveness of the catalyst, due to limitations in the mass and heat transport.
In accordance with this, the present invention relates to a supported cobalt-based catalyst characterized in that the carrier, preferably consisting of at least 80% of aluminum oxide, has an average particle diameter, measured by means of Coulter LS230, ranging from 70 to 250 xcexcm, preferably from 120 to 180 xcexcm, a surface area higher than 175 m2/g, preferably from 185 m2/g to 210 m2/g, and a pore volume higher than 0.35 cm3/g, preferably higher than 0.40 cm3/g, measured with the B.E.T. (Brunauer, Emmett, Teller) method.